Sound-recording apparatus



Feb. 17, 1931.

F. A. MITCHELL SOUND RECORDING APPARATUS Filed Jan'.

2 Shets-Sheet 1 PUG.

FIG. 3

FRANK A INVENTOR um MITCHELL Feb. 17, 1931. T E 1,792,859

SOUND RECORDING APPARATUS Filed Jan. 17, 1,927 2 Sheets-Sheet 2 INVENTOR FRANK ALLEN Mwcnmp V ATTORNEY Patented Feb. 17, 1931 UNITED STATES PATENT FFICE FRANK ALLEN MITCHELL, LONDON, ENGLAND, ASSIGIZOR TO COLUIKBIA IPHONO- GRAPH COMPANY, INC, OF BRIDGEPORT, CONNECTICUT, A COREORATION OF NEW YORK SOUND-RECORDING APPARATUS Application filed January 17, 1827, Serial No. 161,552, and in Great Britain January 27, 1926.

This invention relates to electrical soundrecordiu apparatus by means of which music and other sounds may be recorded upon a so or other form of record-carrying w 7'. di medium, the object of the invention belng to irovide for the exercise of greater control over the movements of the record-engraving stylus point and to enable a greater amount of energy to be applied to the stylus point in order to produce a less distorted and at the same time a louder and better balanced record than has been possible with the mechanisms hitherto adopted.

The invention comprises certain improvements in electro-magnetically operated sound-wave recording apparatus and is intended to he used in combination with a microphone, elcctro-magnetic or other form of transmitter coupled directly or through a suitable amplifying device to the said recording apparatus.

The invention consists in means by which a certain amount of stability is imparted to the stylus bar to prevent sound-wave impulses of excessive amplitude being recorded by the stylus on the record-carrying material.

Th 'nvention also consists in an electromag; ieticallv operated sound-wave recording do i I incorporated therewith addiro-magnetic means by which the ot recorded sound-wave impulses cdetirmined limit are damped 'tude of sound-wave impulses It are undamped by said magani )litu means.

The invention further consists in a soundvrave recording device comprising a movable stvlus holder adapted to be operated by electro iguets a microphone-controlled circuit connected to vary the magnetic flux value of the electric-magnets, and a normalizing circuit connected to said electro-magnets for the purpose clamping the amplitude of sound-war i pulses above a predetermined limit when recorded 'by' said stylus.

ll 0. invention still further comprises other and arrangements hereinafter de- "ith reference to the attached drawn s, in which F is a wiring diagram of an electroi operated sound-wave recorder accordance with the invention, together with its n .rmalizing control circuit and its nicrophone-controlled operating circuit.

Fig. 2 shows a practical construction for an electromagnetic recorder in Fig. 1, together with its connections to the microphone-controlled amplifier and its control potentiometer.

Fig. 3 is a wiring diagram of an alternative system according to the invention in which both the microphone-controlled current and the normalizing current pass through the same coils.

Fig. 4 is a wiring diagram showing the application of a microphone-controlled pushpull thermionic amplifier to a magnetic recorder using horseshoe magnets which are normalized by controlled thermionic valve emission.

Fig. 5 shows a wiring diagram in which use is made of a microphone-controlled pushpull 'hcrmionic amplifier employing valves in parallel, the output being applied to the magnetic recorder by means of two independent sets of connections. Output milliammeters are also included in the circuit.

Fig. 6 is a graph showing the flux value of the electro-magnets in relation to the amplifier output.

Fig. 7 shows the resulting damped waves relative to the amplifier output.

' In carrying the invention into effect in one simple and convenient manner, I may arrange a very light stylus bar 10 to be pivotally mounted on a rigid frame 11 (see Fig. 2), the stylus bar 10 having a sapphire or other suitable engraving point 12 attached thereto in such a position that it is free to engrave a Jill will attract the armature 16 and move the stylus barin the other direction.

When the magnets are energized'equally or when they are not energized the stylus bar is normally maintained in its most suitable position and its movement in relation tothe electro-magnets 13 and 14 is partially con rolled by means of rubber pads or resilient springs 17 in adjustable holders 18 (see Fig. 2) or any other resilient device with which the stylus bar engages. may be used.

The electro-magnet windings 19 and 20-are connected directly or hrough the medium of a wireless or other amplifier (indicated generally by the reference character 21 in Fig. 2) with a microphone or other form of transmitter 22 adapted to be operated and controlled by the sound waves that are to be recorded- The amplifier shown in Fig. 1 comprises a simple thermionic valve 23 having its grid 24 connected to be influenced by themicrophone 22 and its anode 25 connected by means of a transformer 26 to the magnet windings 19 and 20.

For the purpose of obtaining a greater variation in the magnetic flux densities in the electro-magnet. cores 13 and 14 and for the purpose of controlling the maximum amplitude of the stylus bar movements the magnet cores are preferably kept under aslight uniform magnetomotive force which may.

conveniently be adjustable by means of a rheestat. or other suitable regulating device. This may be effected in one convenient manher by providing additional windings 27 and 28 (Figs. 1 and 2) connected with a battery 29 by means of a potentiometer 30 for controlling the normalizing current in the coils 27 and 28. I

To take a specific example of the efiect oi this normalizing current it may be supposed that the two magnet cores 13 and 14 of soft iron are kept magnetized by a direct current supplied by the battery 29 and traversing the coils 27 and 28 which are wound in opposite directions round the. respective cores 13 and 14 so that if a iiuxdensity A is generated due to the current supplied the average magnetic flux density of each core will have a normal value of +A in core 13 for example and A in core 14. p

The electro-magnet coils 19 and 20 connected directly or. indirectly to the micro-- phone 22 or the like on the other hand are 'in both m-agnet cores 13 and 14, the or value being proportional to the sound. wave variatlon.

Thus taking the first half of a sound wave the flux density in the magnet core13 may be the normal flux A plus the superimposed flux of value B so that the total flux is A-l-B, while for the magnet core 14'the total flux will be B A. During this part of the sound wave therefore the magnet 13 has a flux power value of 2B relative to the value of 14 so long as the value 13 does not exceed that of A, the result when B exceeds A. in value being reerred to hereafter.

F or the second half of the sound wave the magnet core 13 has a normal flux +A (as before) with asuperimposed flux B so that the total flux for 13 is A-B. The magnet core 14 on. the other hand: has a normal value A (as before) and superimposed value B giving a total flux for 14 of (Ad-B) thus giving magnet 14 a flux power value of (23) over the magnet 13. Thus, for the superimposed magnetic flux variations of +13 and B I obtain a power. advantage of twice the attractive force due to the superimposed: flux, that is, an attractive force due to +28 or 2B exerted upon the armature which in this case is the recording stylus bar 10' or is attached thereto, and by regulating the normal magnetic flux density the maximum power value will also be controlled, no matter what may be the variations in the superimposed-magnetic flux. Thus, by a very simple calculation it may be shown that the magnetic power advantage alternates from magnet 13 to magnet 14 in tone sympathy with the superimposed alterations and: that the power magnification is directly proportional to the superimposed flux variations B so long as B is equal-to or less than the normal flux value A. By the same calculation it may be shown that any excess in the value B above the value A does not add to the magnetic power difiierence between the two magnets 13 and 14,. but when a superimposed magnetic flux B controlled by the. variable output from the microphone 22 is in excess of the chosen maximum as determined by the normalizing flux A, the polarity of one operating magnet is. reversed. and operates in opposition to the other magnet to prevent excessive movement of the recording stylus.

It will be seen from the above. that by regulating the normal magnetomotiveforce A it is'possihle to control the maximum movement or swing of the recording stylus bar, thus preventing the recording of sound vibrations of large amplitude which usually render the record unfit for use.

3 shows another system of wiring to i1 1 magnetic effect as described T- illustration shows a method by which tl e same magnet coils 31 and 32 are used for carrying both the normalizing current A and for carrying the inicrophono controlled current B.

The normalizing flux A is controlled by the rheostat 33 and in the example passes through the coils 31 and 32 in such a manner that the two adjacent enos of the cores 13 and 14 are of the same polarity.

The output 13 from the microphone-con trolled tram-dormer 26 operates through the condensers and 35 to generate opposing poles in the coils 31 and 32 in such a manner that the magnetic flux in one coil is B +A and in the other coil is B A, the actuation of the stylus bar 10 being the same. as in the previous example.

The recorder shown in 1 is provided with a double. pole magnetic system on each side of the stylus bar 10. One limb of each magnet 13 and 1 1 is )IOVlClGCl with its respective indings 27 and 28 for the normalizing current A from the thermionic valve 36. (the value of which is controlled by adjustment of the grid bias from battery 37) while the other limbs 38 and 39 have the re spcctive windings 19 and 20 for the microphone-controlled current derived from puslrpull thermionic amplifier 420 and 1-1. As before two of the windings (Sr and 23 for example) are opposingly wound and the other two windings (19 and 20) are both wound in the same direction so that while the magnetic flux in one magnet is A B, the n agnetic flux in the othermagnet (14) is Pi -A, and thus the same effect will be produced on the armatures 15 and 16 and stylus holder 10 as in the previous examples.

The magnet limbs or cores 13 and 38. or 1 1 and 39 may be connected and 1-3 by a soft iron bridge or by a non-magi. sable material such as brass for example.

The type of amplifier shown in Fig. l 1 particularly suitable for an clectro-magnerecorder in accordance with this invention. The output from the microphone 2 is passed through any suitable frequency modifying circuit as shown at 14 in Fig. 5, for example) and passes through the transformer 15 the grids 2d of valves 23 and 41 arranged in such a. ma" er that the input from the transfor er L .rded but funct ons as an oscillating not al between the two grids 24 111 such a n'ianner that while the grid in 23 (for example) positive, the grid in ll is negative relative to the control bias applied by the bet tery -16 and resistance 17. i

in mcr that an alternating current is generated in'the output connections to the magnet co 19 and 29011 the recorder, the intensity of each half cycle of the transforn'ier altern' Lions being of equal strength and thus causing the stylus holder 10 and stylus 12 to record each half cycle of the sound waves with equal amplitude.

The necessity for a push-pull circuit of this type will become evident by reference to Fig. 5 which shows two pairs of valves 11 and 4-8, and 49 and 50 arranged in parallel, one pair for amplifying each half cycle of the soundwave impulse.

The output from the valves is connected to the transformer 26 in such a manner that it functions in a similar manner to that shown in Fig. 4, with the exception of a by-path tapping at 51 which connects with the compensating coils 52 and 53 on the magnet cores l3 and 14-.

The output from the transformer 26 is connected to the coils 19 and 20 mounted .on the limbs 38 and 39 of the magnets as before. The normalizing current controlled by the pptentiometer 30 passes through the coils 2x and 28 on the other limbs of the magnets 13 and ll also as before, the magnets 13 and 1d operating, for example, on the stylus holder 10 and armature 15 and 16 in the following C( for example) dependent on the value of the resistance 51 relative to the resistance of the transformer 26.

The other valves 19 and 50 having a much lower emission value than valves 41 and 15% (owing to the negative grid potential momentarily applied thereto) will be treated for the sake of example as having a neutral emission. Therefore, the coil 53 on the other mag net 13 will have no flux value as it is depend out on valve emission and not transformer action for its current supply. The neutralizing coils 27 and 28 have a constant flux value of +A, as explained with reference to the previous examples.

- The total flux values of the two magnetic vstems 13 and 1 1 would. be as follows: The

flux in core 13 and 38 would be A+B C, and

' of QB-l-C over the-flux value of 14- as long as the value of B is less than the, value A. During the other half cycle of the soundwave impulse the conditions would be reversed, thus causing the stylus bar to be rocked in the opposite direction.

Should the value of B exceed. the value of A+C, the polarity of one of the magnets would be reversed and the flux value of 2B+C would beco1ne2A+C. This can be checked by the simple calculation of (A+B+C) (AB+O),

as explained above, it being remembered that it is merely the flux value of the magnet that we are concerned with, and not the direction of the resulting flux. V r V Milliammeters are provided at 54, and 56 in the circuits for the purpose of adjusting the relative values of A, B and C.v

The effect of a magnetic circuit according to the above arrangement on the power applied to operate the recording stylus will bev The effect on the sound wave as recorded will be seen on reference to Fig. 7, in which 61 is the microphone-controlled output which should be approximately equal in relative values to the original sound waves operating the microphone, and in which 62 represents the controlled sound waves actually record ed on the wax or other record-carrying material.

It will be seen that the sound waves having small amplitude are undamped while those waves having an amplitude greater than the control normalizing current (denoted by lines 63) are considerably damped.

In some cases permanent magnets may be used in place of soft iron cores or armatures, or they may be used in combination with electro-magnets to obtain the desired result. Further, the normalizing magnetomotive force may be applied to maintain the opposing magnets of opposite or of similar polarities and when normally the magnets areof similar polarities the superimposed magnetomotive variations would be so applied to the magnets as to be positive on one side and negative upon the other but again by a simple calculation it may be shown that the results will be the same as in the example hereinbefore given where the. normalizing magnetomotive force maintains the-magnets of opposite polarities.

Further, I may arrange to produce superimposed flux variations not by alterations as above described but by simple changes in the values of a direct current, in which case it may be preferable that a portion or the whole ot the normalizing current should pass through the microphone or similarly controlled circuit.

Moreover, the invention is not limited to the foregoing details which are given by way of example only, as I may modify the number and method of arrangement of the electromagnets and means adopted for connecting the same-directly or indirectly to a micro phone or other like transmitter and the arrangements provided for maintaining the normal flux density in the electro-magnets depending upon any practical requirements that may have to be fulfilled.

Having now particularly described and. ascertained the nature of my said invention, and in what manner the same is to be performed, I declare that what I claim is 1. In a sound wave recording device, a movable armature, a microphone circuit, electromagnets energized from said circuit and associated with said armature, the coils of said Telectromagnets being wound in opposite directions whereby adjacent ends of the cores of the magnets will have opposite polarity, other coils on the cores of said magnets, said other coils wound in the same direction whereby said coils tend to magnetize each core with the same polarity, and an electrical circuit including said other coils, said other coils acting to impress an additional magnetic flux on said electromagnets where by to damp sound wave impulses above a predetermined limit and leave undamped sound wave impulses below said limit.

2. In an electromagnetic sound wave recording device, a movable armature, a micro-' phone circuit, electromagnets energized from said circuit and disposed to operate said armature, the coils of said electromagnets wound in opposite directions whereby adjacent ends of the cores of the magnets will have opposite polarity, a secondv circuit, other coils in the second circuit and wound in like direction, and said other coils disposed to impress a magnetic flux on the cores of said electromagnets whereby to damp sound Wave im pulses above a predetermined limit and leave undamped sound wave impulses below said limit.

3.ln an electromagnetically operated sound wave recording means, a pivoted stylus holder, electromagnetsfor operating said stylus holder, the cores of each of said electromagnets being in the form of a horseshoe and disposed to operate one on each side of the stylus holder pivot, means controlling said electromagnetic recording means whereby to damp sound wave impulses above a predeterminedlimit and leave undamped sound wave impulses below said limit, an electrical circuit including said last means,'and means in said electrical circuit to control said means and thereby vary said predetermined limit.

4. A sound wave recording device comprising a movable stylus holder, operating electromagnets for moving the same, a micr0- phone controlled circuit connected to vary the magnetic flux value of said electromagnets, and means for applying a polarizing flux to said electromagnets for the purpose of damping the amplitude of sound wave impulses above a predetermined limit when recorded by a stylus in said holder, said polarizing flux being so applied to the operating electromagnets that the microphone controlled fluxes are added to on one-half cycle of a sound vibration and subtracted from on the other half cycle of said sound virbation.

5. A sound Wave recording device comprising a movable stylus holder, electromagnets for operating the same, a microphone controlled circuit connected to vary the magnetic flux value of said electromagnets, means to apply a polarizing flux to said electromagnets for the purpose of damping the amplitude of sound wave impulses above a predetermined limit and leaving substantially un damped sound Wave impulses below said limit when recorded by said stylus, and means for varying the value of the applied polarizing flux as required and while operating.

6. In a sound wave recording device, a movable stylus holder, an electromagnetic system including a pair of electromagnets positioned to move the stylus holder to record sound wave impulses, means for applying a constant polarizing flux to the cores of said electromagnets to damp the amplitude of sound wave impulses above a predetermined limit which are to be recorded and to leave undamped sound wave impulses below said limit, and a thermionic valve controlling the applied constant polarizing flux density.

7. In a sound Wave device, a pivoted stylus holder, a main electrical circuit, electromagnets in said circuit adapted to move the stylus holder about its pivot in accordance with sound wave impulses in said circuit, an auxiliary circuit, coils within said auxiliary C11- cuit and wound in the same direction and disposed to polarize the cores of the electromagnets of the main circuit, means for varying the value of the current in the auxiliary circuit, the imposing of the polarizing flux on the electromagnets effecting said magnets to limit the amplitude of vibration of the stylus holder under the influence of the magnets to a value determined by the value of the current in the auxiliary circuit, and the value of the fluxes of the auxiliary and main circuits being additive on one-half cycle of a sound vibration and subtractive on the other half cycle thereof.

8. In an electromagnetically operated sound wave recording device, a recording stylus, electromagnets for operating said stylus, a microphone controlled circuit for impressing a variable magnetic flux on said magnets, an auxiliary circuit, means in said auxiliary circuit for impressing a polarizing flux on said magnets, said polarizing flux o erating to reverse the polarity of one 0 said magnets and have it operate at opposition to another of said magnets to prevent excessive movement of the recording stylus on the variable magnetic flux exceeding a chosen maximum as determined by the polarizing flux density.

9. A step in the method of recording sound, said step consisting in electromagnetically damping the amplitude of sound wave impulses above a predetermined limit and leaving undan'iped the amplitude of sound wave impulses below said limit.

10. A step in the method of recording sound, the step consisting in damping the amplitude of sound wave impulses above a predetermined limit and leaving undamped the soimd wave i npulses below said limit.

11. The in the method of recording sound consisting in electromagnetically damping the amplitude of sound wave impulses above a predetermined limit, varying said limit to meet varying conditions and leaving rind-amped the amplitude of sound wave impulses below said limit.

12. The method of recording sound consisting in electromagnetically operating an armature in accordance with impulses delivered to a circuit by a microphone and electromagnetically damping the operation of the armature to damp sound wave impulses above a 1 redetermined limit and leaving the armature undamped when recording sound wave impulses below said limit.

Signed at London, England, this 29th day of December, 1926.

FRANK ALLEN MITCHELL. 

